Thanks. Could you please clarify to me, if possible, using some practically relevant example, how fault notification for a set of undefined fault types that may vary from MPI implementation to implementation differs from the equally abstract MPI_Checkpoint/MPI_Restart that semantically clearly prepare the MPI implementation at hand for the checkpoint action done by the checkpointing system involved, and then semantically clearly recover the MPI part of the program after the system restore? -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Greg Bronevetsky Sent: Friday, February 13, 2009 7:03 PM To: MPI 3.0 Fault Tolerance and Dynamic Process Control working Group; MPI 3.0 Fault Tolerance and Dynamic Process Control working Group Subject: Re: [Mpi3-ft] Communicator Virtualization as a step forward At 09:55 AM 2/13/2009, Supalov, Alexander wrote:

Thanks. I guess there are several ways to deal with this situation:

- Hope that market forces will make all MPIs provide a reasonable level of FT support that will stabilize with the time. - Make certain promises in the standard and let people claim FT compliance to this level and thus provide certain level of support.

Introduction of FT support is akin to the thread support introduction. In that case the Forum was able to determine several reasonable levels that found acceptance with the users, and now we see that mixed mode programs are starting to appear in substantial numbers.

I would argue that the FT support in MPI-3 should attempt to do something comparable. Providing a variable and unpredictable level of FT support, which is how the initial description came across to me, may not be good enough for people to take the plunge. Its easier to do with threads because everybody knows what threads mean. With low-level failures there is no standard taxonomy that we can refer to when making the specification. One way to approach the problem is to lead by example. There is work at ORNL to implement the current FT proposal and it will probably become the reference implementation. In the future we'll be able to point to this implementation as well as FT-MPI to say what a reasonable level of support looks like.

In some sense, the discussion on this topic mirrors the discussion on the checkpoint/restart. There I heard arguments that since we cannot define what this may possibly mean down in the MPI, and hence we cannot simply do with the MPI_Prepare_for_checkpoint & MPI_Recover_after_restart calls that would be basically implementation specific (in MPI and checkpointing system sense).

Here we say that we cannot define anything tangible to introduce the FT support levels, but still we are going ahead with introducing FT into the MPI-3, at some unfathomable level, in full hope that life will fix things up somehow.

Do you notice some kind of discord here? I seem to.

The comparison is apt but incomplete. The problem with checkpoint support is that the calls MPI_Prepare_for_checkpoint & MPI_Recover_after_restart will have little semantic meaning. On the other hand, the fault notification API will have clear meaning but does not define which subset of low-level failures fall into the recoverable bucket and which into the non-recoverable bucket. Not defining the buckets is much closer to not defining the communication latency than to not defining what it means to make MPI "checkpoint-ready". Greg Bronevetsky Post-Doctoral Researcher 1028 Building 451 Lawrence Livermore National Lab (925) 424-5756 [email protected] _______________________________________________ mpi3-ft mailing list [email protected] http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-ft --------------------------------------------------------------------- Intel GmbH Dornacher Strasse 1 85622 Feldkirchen/Muenchen Germany Sitz der Gesellschaft: Feldkirchen bei Muenchen Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer Registergericht: Muenchen HRB 47456 Ust.-IdNr. VAT Registration No.: DE129385895 Citibank Frankfurt (BLZ 502 109 00) 600119052 This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies.